Thanks @eskimo. The error is NSXPCConnectionInterrupted (4097). The UI app is not sandboxed, it will be deployed with Developer ID for internal use. The call is coming from a SwiftUI Button action that the user taps on, and works correctly for functions that don't have a reply block.

Correction, we are now getting the NSCocoaErrorDomain Code=4099 error. But at some point yesterday it was 4097.

@eskimo I think I got it. NSXPCConnection invalidate() was being called too early in the client. Still have some learning to do for managing the connection state.

Follow up with solution to the OTA installation error "Cannot connect to...". It was not due to the distribution certificate being for iOS. visionOS required the platform-identifier key in the manifest.plist to be in the assets section. Our platform-identifier has always been in the metadata section but visionOS seems to be more picky.